Polyethylene is one of the most frequently used commercial polymers. It can be prepared by a couple of different processes. Polymerization in the presence of free-radical initiators at elevated pressures was the method first discovered to obtain polyethylene and continues to be a valued process with high commercial relevance for the preparation of low density polyethylene (LDPE). LDPE is a versatile polymer which can be used in a variety of applications, such as film, coating, molding, and wire and cable insulation. There is consequently still demand for optimizing the processes for its preparation.
A normal set-up for a high pressure LDPE reactor consists essentially of a set of two compressors, a primary and a high pressure compressor, a polymerization reactor and two separators for separating the monomer-polymer mixture leaving the tubular reactor, wherein in the first separator, the high pressure separator, the ethylene separated from the monomer-polymer mixture is recycled to the ethylene-feed between the primary compressor and the high pressure compressor and the ethylene separated from the mixture in the second separator, the low pressure separator, is added to the stream of fresh ethylene before it is fed to the primary compressor. Common high pressure LDPE reactors are either tubular reactors or autoclave reactors. Both types of reactors have very often more than one injection point for initiators, thus creating multiple reaction zones.
A key factor to control the polymerization conditions of each reaction zone is the amount and the nature of the used initiator. Adding initiator starts the polymerization reaction, which is strongly exothermic. Accordingly, the temperature rises, which not only influences the polymerization reaction but also the decomposition of the initiator. In view of this complex interdependency it has been proven to be advantageous to use mixtures of different initiators. Since however the polymerization conditions of different reaction zones differ it is further necessary to use different initiator mixtures in different reaction zones. As a consequence, appropriate initiators or initiator mixtures have to be selected for each grade and each reaction zone. This can be done based on experience and experimental date. This can however also be supported by computer-aided tools as described in WO 2004/078800, which refers to a method of controlling the process for the continuous free-radical polymerization of ethylene homopolymers or copolymers by selecting initiator mixtures with respect to minimum initiator costs.
Initiators for starting a free-radical-initiated polymerization like organic peroxide have to be handled with care since such compounds are noxious and thermally unstable. If heated above a certain temperature they will decompose in a runaway reaction. Accordingly, storage and handling of such chemicals need special precautions.
For carrying out the polymerization in high pressure reactors with initiator mixtures, it is common practice to premix the initiators, optionally with additional solvents, and meter such a mixture to the reactor. The number of mixtures which has to be prepared is limited and equals at the most the number of reaction zones. Usually, the components of the initiator mixtures are withdrawn from some storage facilities, combined in a special mixing vessel and then transferred to a reservoir, from which the initiator mixtures are metered to the respective reaction zone. However, since each mixing operation requires special attention of the operators, the number of mixing operations in a given time period should be as low as possible and moreover, if a larger quantity of material is mixed, small divergences in the amounts result only in relatively small variations of the composition. That means, the initiator mixtures are generally prepared in a quite large scale. Although this method provides a reliable way for entering initiator mixtures into a reactor, there are still drawbacks. The method is labor intensive and it lacks flexibility. There is no room to react if parameters of the reaction vary, for example, the composition of the ethylene feed, the temperature of the cooling water, the cooling behavior in the high pressure recycle or different conditions of the high pressure compressor. Moreover, in case of a desired grade change it is either necessary to wait with the transition until the whole quantity of initiator mixture is consumed or the remaining quantity of initiator mixture has to be disposed of, which is not only costly but also ecologically unfavorable and labor intensive. Furthermore, if an initiator mixture is kept too long, there is always the chance that this initiator mixture deteriorates. The over-all activity can diminish or a phase-separation may occur. Causes could be, for instance, that by-products of different initiators react with each other or that they react with other initiators.